The present invention relates to a switching power supply circuit suitable as a power supply for a television receiver, for example.
A color television receiver having a cathode-ray tube (hereinafter abbreviated to a CRT) for image display, for example, generally uses a power supply circuit formed by a horizontal deflection circuit for deflecting an electron beam emitted from an electron gun within the CRT in a horizontal direction and a switching power supply of the soft switching type formed by a current resonance type converter.
FIG. 9 shows configuration of a horizontal deflection circuit and its peripheral circuit provided in a television receiver.
The switching power supply 10 shown in FIG. 9 is a DC-to-DC converter for performing switching operation on a direct-current voltage inputted thereto and converting the direct-current voltage into a direct-current voltage having a specified voltage level for output.
A rectifying and smoothing circuit is provided before the switching power supply 10. The rectifying and smoothing circuit rectifies and smoothes a commercial alternating-current power VAC to thereby provide a direct-current voltage Ei. The direct-current voltage Ei is then inputted to the switching power supply 10.
The switching power supply 10 outputs a direct-current output voltage E01 converted into a specified voltage level and the like.
In this case, the direct-current output voltage E01 is a voltage for driving the horizontal deflection circuit of the television receiver, and is 135 V, for example.
A horizontal output circuit 20 generates a horizontal deflection current IDY for scanning an electron beam emitted from an electron gun in a CRT in a horizontal direction, and also generates a flyback pulse for generating a high voltage in a high-voltage generating circuit 40, which will be described later.
Thus, a pulse voltage in synchronism with a horizontal synchronizing signal fH of a video signal is inputted from a horizontal driving circuit not shown in the figure to a base of a horizontal output transistor Q11 of the horizontal output circuit 20.
A collector of the horizontal output transistor Q11 is connected to a secondary-side output terminal E01 of the switching power supply 10 via a primary-side winding N11 of a flyback transformer FBT. An emitter of the horizontal output transistor Q11 is grounded.
A damper diode D11, a horizontal flyback line capacitor Cr11, and a series connection circuit formed by a horizontal deflection yoke Hxc2x7DY, a horizontal line correction coil HLC, and an S-shape correction capacitor CS1 are each connected in parallel with the collector and emitter of the horizontal output transistor Q11.
In the horizontal output circuit 20 thus formed, capacitance of the horizontal flyback line capacitor Cr11 and a leakage inductance component of the primary-side winding N11 of the flyback transformer FBT form a voltage resonance type converter.
The pulse voltage inputted from the horizontal driving circuit not shown in the figure causes the horizontal output transistor Q11 to perform switching operation, whereby a horizontal deflection current IDY having a sawtooth waveform flows through the horizontal deflection yoke Hxc2x7DY. During the off period of the horizontal output transistor Q11, a relatively high pulse voltage V11 is generated across the horizontal flyback line capacitor Cr11 as a result of resonance operation by inductance LDY of the horizontal deflection yoke Hxc2x7DY and the capacitance of the horizontal flyback line capacitor Cr11 and the effect of the damper diode D11.
Incidentally, the horizontal line correction coil HLC and the S-shape correction capacitor CS1 correct the horizontal deflection current IDY, for example, to thereby correct distortion of an image displayed on the screen of the CRT.
The high-voltage generating circuit 40 enclosed by alternate long and short dashed lines comprises the flyback transformer FBT and a high-voltage rectifying and smoothing circuit, for example. The high-voltage generating circuit 40 steps up the flyback pulse voltage V11 generated in the horizontal output circuit 20 to thereby generate a high voltage whose level is equivalent to that of an anode voltage of the CRT.
The primary-side winding N11 is wound on the primary side of the flyback transformer FBT, and five step-up windings NHV1, NHV2, NHV3, NHV4, and NHV5 are divided and wound by slit winding or layer winding on the secondary side of the flyback transformer FBT.
Also, tertiary windings N12 and N13 are wound in a state of being closely coupled to the primary-side winding N11 on the primary side of the flyback transformer FBT.
In this case, the step-up windings NHV1 to NHV5 are wound in a winding direction such that the step-up windings NHV1 to NHV5 are of opposite polarity from the primary-side winding N11. The tertiary windings N12 and N13 are wound such that the tertiary windings N12 and N13 are of the same polarity as the primary-side winding N11.
A starting point of the primary-side winding N11 is connected to the secondary-side output terminal E01 of the switching power supply 10, while an ending point of the primary-side winding N11 is connected to the collector of the horizontal output transistor Q11.
Ending points of the step-up windings NHV1 to NHV5 are connected with anodes of high-voltage rectifier diodes DHV1, DHV2, DHV3, DHV4, and DHV5, respectively.
A cathode of the high-voltage rectifier diode DHV1 is connected to a positive terminal of a high-voltage capacitor CHV, and cathodes of the high-voltage rectifier diodes DHV2 to DHV5 are connected to starting points of the step-up windings NHV1 to NHV4, respectively.
Specifically, a half-wave rectifier circuit of the so-called multi-singular type is formed on the secondary side of the flyback transformer FBT by series connection of five half-wave rectifier circuits: the step-up winding NHV1 and the high-voltage rectifier diode DHV1; the step-up winding NHV2 and the high-voltage rectifier diode DHV2; the step-up winding NHV3 and the high-voltage rectifier diode DHV3; the step-up winding NHV4 and the high-voltage rectifier diode DHV4; and the step-up winding NHV5 and the high-voltage rectifier diode DHV5.
Thus, on the secondary side of the flyback transformer FBT, the five half-wave rectifier circuits rectify currents induced in the step-up windings NHV1 to NHV5 and store the resulting currents in the high-voltage capacitor CHV, whereby a high direct-current voltage EHV whose level is equivalent to five times the voltages induced in the step-up windings NHV1 to NHV5 is obtained across the high-voltage capacitor CHV. The high direct-current voltage EHV obtained across the high-voltage capacitor CHV is used as the anode voltage of the CRT, for example.
Incidentally, an induced voltage stepped up to 6 KV is obtained in each of the step-up windings NHV1 to NHV5, and an anode voltage of 30 KV is obtained as the high direct-current voltage EHV.
The primary-side winding N11 of the flyback transformer FBT is provided with a tap. A half-wave rectifying and smoothing circuit formed by a rectifier diode D03 and a smoothing capacitor C03 rectifies and smoothes a positive pulse voltage obtained from the tap to thereby supply a direct-current output voltage E03 from across the smoothing capacitor C03. The direct-current output voltage E03 has a voltage level of 200 V, for example, and is supplied to a cathode of the CRT via a video signal amplifier not shown in the figure.
A rectifying and smoothing circuit formed by a rectifier diode D06 and a smoothing capacitor C06 and a rectifying and smoothing circuit formed by a rectifier diode D07 and a smoothing capacitor C07 rectify and smooth a negative pulse voltage obtained from the tertiary winding N12 wound on the primary side of the flyback transformer FBT, and thereby supply direct-current output voltages E06 and E07 from across the smoothing capacitors C06 and C07, respectively. The direct-current output voltages E06 and E07 have voltage levels of +15 V and xe2x88x9215 V, respectively, and are used as driving voltage of a vertical deflection circuit not shown in the figure.
A rectifying and smoothing circuit formed by a rectifier diode D08 and a smoothing capacitor C08 rectifies and smoothes a negative pulse voltage obtained from the tertiary winding N13 to thereby supply a direct-current output voltage E08 from across the smoothing capacitor C08. The direct-current output voltage E08 is 6.3 V, for example, and is used as voltage for a heater of the CRT.
FIGS. 10A, 10B, 10C, 10D, and 10E show operating waveforms of parts of the circuit shown in FIG. 9.
Since the pulse voltage in synchronism with the horizontal synchronizing signal fH of a video signal is inputted to the base of the horizontal output transistor Q11 in the circuit of FIG. 9, switching frequency of the horizontal output transistor Q11 coincides with frequency of the horizontal synchronizing signal fH. The horizontal output transistor Q11 is turned on during a horizontal scanning period Tt (51.5 xcexcs), and is turned off during a horizontal flyback period Tr (12 xcexcs). Hence, a period TH of 63.5 xcexcs, which is a sum of the horizontal scanning period Tt and the horizontal flyback period Tr, coincides with a cycle of the horizontal synchronizing signal fH.
In this case, as a result of switching operation of the horizontal output transistor Q11, a primary-side current I11 having a waveform as shown in FIG. 10B flows through the primary-side winding N11 of the flyback transformer FBT, and a horizontal deflection current IDY having a waveform as shown in FIG. 10C flows through the horizontal deflection yoke Hxc2x7DY. A rectified current I3 having a waveform as shown in FIG. 10E flows through the rectifier diode D03 via the tap provided to the primary-side winding N11.
In this case, as shown in FIG. 10A, a voltage V11 across the horizontal flyback line capacitor Cr11 connected in parallel with the collector and emitter of the horizontal output transistor Q11 is at a zero level during the on period Tt of the horizontal output transistor Q11, and forms a flyback pulse voltage V11 of about 1200 Vp, for example, during the off period Tr of the horizontal output transistor Q11 as a result of resonance operation by the inductance component LDY of the horizontal deflection yoke Hxc2x7DY and the capacitance of the horizontal flyback line capacitor Cr11.
Thus, the high-voltage generating circuit 40 steps up a positive pulse voltage applied to the primary side of the flyback transformer FBT, which results from the flyback pulse voltage V11, whereby different direct-current output voltages having specified voltage levels are obtained from the step-up windings NHV1 to NHV5 on the secondary side and the tertiary windings N12 and N13.
As shown in FIG. 10D, a pulse voltage V3 of about 200 Vp, for example, is generated across the smoothing capacitor C03 during the off period Tr of the horizontal output transistor Q11. The rectifier diode D03 and the smoothing capacitor C03 rectify and smooth the pulse voltage V3 to thereby provide the direct-current output voltage E03.
The flyback transformer FBT of the high-voltage generating circuit 40 in the circuit of FIG. 9 converts the direct-current voltage E01 inputted from the switching power supply 10 into the high direct-current voltage EHV at a power conversion efficiency of about 85%. Therefore, when high-voltage load power is 60 W, for example, a power loss of about 9 W occurs.
In addition, the high-voltage generating circuit 40 subjects peak values of the currents induced in the secondary-side step-up windings NHV1 to NHV5 by the positive pulse voltage inputted to the primary-side winding N11 of the flyback transformer FBT to half-wave rectification, and thereby provides the high direct-current voltage EHV.
In this case, however, conduction angles of the high-voltage rectifier diodes DHV1 to DHV5 are narrow, and equivalent power supply impedance is high. Therefore, the voltage level of the high direct-current voltage EHV is easily affected by variation in the high-voltage load.
When the circuit is applied to a television receiver having a CRT screen size of 34 inches or more, highlighting at the highest brightness on the screen of the CRT requires a beam current IHV of 2 mA or more to be supplied to the anode of the CRT, for example. Hence, when it is supposed that the high direct-current voltage EHV supplied to the anode has a voltage level of 30 KV, for example, a power of 60 W (30 KVxc3x972 mA) is required as the high-voltage load power to be applied to the high-voltage generating circuit 40 during highlighting.
Thus, the high-voltage load power supplied from the high-voltage generating circuit 40 to the anode of the CRT may be considered to be varied at least from 0 W (IHV=0 mA) to 60 W (IHV=2 mA).
In this case, when it is supposed that the beam current IHV of 2 mA flows through the anode of the CRT and the high direct-current voltage EHV has a voltage level of 30 KV when the high-voltage load power of the high-voltage generating circuit 40 is 60 W, the high direct-current voltage EHV raises its voltage level to 32.5 KV, for example, under load conditions where the high-voltage load power of the high-voltage generating circuit 40 is 0 W. Thus, when the circuit is applied to an actual television receiver or the like, a voltage variation range xcex94EHV of the high direct-current voltage EHV within an actually employed range of the high-voltage load power (0 W to 60 W) is about 2.5 KV. This results from a voltage drop at the high-voltage rectifier diodes DHV1 to DHV5 forming the high-voltage generating circuit 40 due to variation in the high-voltage load power applied to the high-voltage generating circuit 40.
When the horizontal deflection current IDY has a constant current value, for example, such variation in the voltage level of the high direct-current voltage EHV results in a change in horizontal amplitude of the electron beam outputted from the CRT. Therefore, the horizontal output circuit 20 in an actual television receiver needs to be provided with a zooming correction circuit and the like for correcting the current value of the horizontal deflection current IDY so that the variation in the high direct-current voltage EHV will not change the horizontal amplitude of the electron beam.
Moreover, because of its structure, for example leakage inductance of the secondary-side step-up windings NHV1 to NHV5, the flyback transformer FBT causes ringing in timing in which the level of the voltage induced in the step-up windings NHV1 to NHV5 becomes negative.
When the ringing component is superimposed on the primary-side current I11 shown in FIG. 10B, which flows through the primary side of the flyback transformer FBT, raster ringing, a curtain pattern and the like are produced on the left edge of the screen of the CRT.
Therefore, an actual television receiver needs to be provided with some measures to prevent the raster ringing and the curtain pattern.
It is thus an object of the present invention addressing the problems described above to provide a switching power supply circuit for rendering a high direct-current voltage outputted from a flyback transformer constant.
To achieve the above object, according to an aspect of the present invention, there is provided a switching power supply circuit comprising: switching means including a switching device for interrupting a direct-current input voltage inputted thereto for output; an isolation converter transformer for transmitting an output on a primary side thereof to a secondary side thereof, the isolation converter transformer including a primary-side winding wound on the primary side and a secondary-side winding wound on the secondary side, and the isolation converter transformer having a desired degree of coupling to loosely couple the primary-side winding and the secondary-side winding to each other; a primary-side parallel resonant circuit formed by connecting a primary-side parallel resonant capacitor in parallel with the primary-side winding of the isolation converter transformer for converting operation of the switching means into voltage resonance type operation; a secondary-side parallel resonant circuit formed by connecting a secondary-side parallel resonant capacitor in parallel to the secondary-side winding; a step-up transformer for transmitting a resonance voltage inputted to a primary side thereof to a secondary side thereof and thereby supplying from the secondary side a stepped-up voltage obtained by stepping up the resonance voltage, the step-up transformer including a primary-side winding wound on the primary side and a secondary-side winding wound on the secondary side; a series resonant capacitor inserted between the secondary-side winding of the isolation converter transformer and the primary-side winding of the step-up transformer for converting primary-side operation of the step-up transformer into resonance operation, the series resonant capacitor inputting the resonance voltage obtained from the secondary-side winding of the isolation converter transformer to the primary side of the step-up transformer; and high direct-current voltage generating means for performing rectifying operation on the stepped-up voltage obtained on the secondary side of the step-up transformer and thereby providing a high direct-current voltage.
With this configuration, the negative resonance voltage outputted from the secondary side of the isolation converter transformer forming the switching power supply circuit of the complex resonance type is inputted to the primary side of the step-up transformer via the series resonant capacitor. Therefore, it is possible to obtain the high direct-current voltage necessary for horizontal deflection of a television receiver, for example, without intervention of a horizontal deflection circuit system. Furthermore, in this case, it is possible to render the waveform of the resonance voltage inputted to the primary side of the step-up transformer substantially sinusoidal.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.